Can not add new books to calibre libs

Hello,

I use Calibre 3.41.3
As of today I am suddenly unable to add new books to my callibre libraries.

I have tried two different libraries.

I have tried all known methods: drag and drop, create empty epub file from within Calibre, add files from directory etc.

Whatever I do Calibre freezes and I need to use task manager to shut down calibre.

Does anybody have an idea what could be going on?

I have had the drag and drop problem before when I made calibre run as administrator. When I run calibre as administrator the drag an drop doesn't work anymore.
But that is not what is going on now. I can not add any new book to any library with any known method. Calibre just freezes.